In the development of internal combustion engines, the issues of fuel consumption and exhaust gas emission limits are assuming increasing importance. These operating parameters may be influenced by various measures or manipulated variables, for example exhaust gas recirculation, moving the charge motion control valve, adjusting the camshaft, modifying the valve train, and/or regulating the engine temperature, or regulating the coolant temperature and/or the coolant flow rate. All these manipulated variables also have an influence on the knock limit of the engine, and therefore on the optimum ignition angle as well.
For this reason, the base ignition angle is determined in practice in multiple steps. Based on the instantaneous engine speed and load, an initial ignition angle is first determined. This initial ignition angle is usually read out from an appropriate characteristic map. To determine the base ignition angle, an ignition angle offset is then added to the initial ignition angle for each manipulated variable, i.e., for each of the above-mentioned functionalities, to enable operation of the engine under all operating conditions at the most optimum efficiency. The ignition angle offsets for the individual manipulated variables are usually determined from appropriate characteristic maps as well. The base ignition angle thus determined forms the starting point for a downstream cylinder-specific knock control, thereby allowing retardation of the base ignition angle when knocking has been detected.
In practice, the above-described method has proven to be problematic in several respects. For manipulated variables which are calculated using a model, dynamic changes frequently result in errors in determining the manipulated variable. This results in erroneous mapping to the corresponding characteristic map for the ignition angle offset, and thus results in less than optimum determination of the base ignition angle. The exhaust gas recirculation rate and the engine temperature are examples of manipulated variables which may be calculated using a model. The model calculations for determining the engine temperature under alternating load or a change in the coolant flow rate and/or coolant temperature have often proven to be inaccurate. One reason is the slow transient phenomena which occur during temperature changes. In addition, changes in the coolant flow rate have a strong non-linear effect on the component or engine temperature. It is possible that the known ignition angle pilot control may not take these circumstances into consideration.
As previously stated, in the known ignition angle pilot control each manipulated variable requires its own characteristic map for the corresponding ignition angle offset. As a consequence, the complexity of data recording and storage increases with the number of manipulated variables to be considered. Data recording for characteristic maps is susceptible to errors based on the sheer quantity of data alone. Furthermore, in some cases data recording is also very time-intensive, such as in the case of the ignition angle offset resulting from dynamic changes in the engine temperature, or the coolant flow rate and the coolant temperature.